The present invention relates generally to antennas suitable for devices for implantation in a human or animal body and to associated apparatus and methods. The invention has particular although not exclusive relevance to antennas used with implantable medical devices, such as but not limited to pacemakers and neurostimulation devices.
Implantable medical devices (IMDs), for human or animal bodies are now ubiquitous. Annually, more than one million pacemakers, more than two hundred thousand defibrillators, and more than one hundred and fifty thousand neurostimulation devices for pain management, epilepsy, Parkinson and many other indications are implanted each year.
Such IMDs often utilize wireless (radio frequency, ‘RF’) technology to enhance the versatility of implantable medical devices by allowing remote monitoring of such IMDs and the optimization of treatments using them. The integration of wireless technology with IMDs represents a significant challenge because of the substantial, inter-related and often conflicting, design constraints placed on IMDs including, for example constraints related to: size; power consumption/efficiency; reliability; durability; operating frequency; bio-compatibility; patient safety; and/or the like.
For example, whether fitted sub-cutaneously or within the peritoneal cavity, IMDs are necessarily small, to allow them to fit within the appropriate pocket in a patient's body, with sizes ranging from a few millimeters (mm) to few tens of centimeters (cm). The size of the battery generally defines the size of the IMD and hence constraints on the size of the IMD inherently limit the choice of power supply. The electronics and the battery are generally enclosed in a hermetically sealed titanium can with a Tecothane or epoxy header.
One of the main challenges facing wireless IMD design is that of antenna design, not least because the anatomical distribution of, together with the different electrical properties and configuration of, the various tissues of a human or animal body, can significantly affect the performance of the antenna. Specifically, the necessary small size of the antenna and the proximity of the antenna to lossy body tissue can result both in signal attenuation and antenna detuning by the local body tissue.
Known antennas, in devices such as pacemakers and Neurostimulation devices reside in a header of the implant. The majority of known IMD antennas are either loops or planar inverted F antennas (PIFA). Loops are current fed antennas and produce primarily a magnetic field (transverse electric (TE) mode). PIFAs, on the other hand, are quarter wavelength (or some multiple of a quarter wavelength), voltage fed, dipole antennas that generate both an electric and a magnetic field. Both these antennas have a single dominant current mode.
Power radiated by antennas is generally complex in nature having a real element and a reactive (or ‘imaginary’) element. The real power leaves the antenna and never returns, whereas the reactive power tends to bounce around about a fixed position (within a half wavelength called the radiansphere) of the antenna and interacts with the antenna and the surrounding environment, thereby affecting the antenna's operation. Generally, antenna design treats a human or animal body as a lossy load that detunes the antenna and therefore seeks to minimize the resonant, non-radiating reactive energy around the antenna, and keep such energy away from tissue, thereby reducing the detuning effects such that the antenna can operate at the desired frequency without the need for retuning. For higher power medical applications, such containment of the resonant energy is seen to be beneficial because it can minimize local RF heating effects that could otherwise cause bio-compatibility issues (e.g. where a local temperature increase of only a few degrees can affect the development and metabolism of cells adjacent to an implant).